The present invention relates to electric machines such as electrical power generators or motors, and in particular to a non-metallic cylinder that restrains both a body portion and end turn portions of a rotor winding.
Conventionally, a rotor of an electric machine has a cylindrical shape including a body portion which extends parallel to the rotor""s axis of rotation and two end faces which axially oppose each other. The rotor winding includes a body portion which is restrained in the body portion of the rotor and end turn portions which extend axially beyond the end faces of the rotor. The body portion of the rotor windings are restrained in the body of the rotor by metallic wedges and the end turn portions of the rotor winding are restrained by high strength retaining rings made of non-magnetic steel. The rotor winding is thus restrained against the centrifugal forces resulting from the rotation of the rotor through the use of the retaining rings in the end turn area and the wedges in the body portion of the rotor. The retaining rings employed in the end turn area typically account for approximately 15% of the total cost of the rotor. Ironically, the mass of the retaining rings themselves result in approximately 60% of the stress produced on the rings. This mass thus necessitates the material forming the retaining rings to have a high strength and thus results in a high cost.
It has been recognized by those skilled in the art that non-metallic materials, such as graphite filament wound epoxy composites, have the necessary tensile strength to restrain the rotor end turn portions against the forces (e.g., centrifugal forces) resulting from the rotation of the rotor. Furthermore, because the graphite filament wound epoxy composite materials typically have a strength to weight ratio of six times that of steel, the stress produced by their own mass is significantly lower. There would thus be a considerable cost savings if a non-metallic material were used in place of the high strength steel currently employed.
Prior attempts to provide a graphite filament wound epoxy composite in place of high strength steel to restrain the end turn portions of the rotor winding encountered numerous difficulties with a ring locking design. The difficulties focused on stress risers at the locking key and the ability of the material to accommodate stress transverse to the filament axis. The designs to overcome these difficulties resulting from the use of graphite filament wound epoxy composites for restraining the winding end turn portions added cost and complexity that negated the cost savings provided by the material.
It would thus be beneficial to provide a single integral device that provides the functions provided by both the retaining rings and the body wedges and to avoid the difficulties encountered with the locking key arrangement of earlier attempts. It would be beneficial to eliminate the use of body wedges to achieve cost savings and increase the amount of copper in each slot of the rotor. It would be further beneficial to eliminate the use of slot teeth which act in concert with the body wedges to restrain the body portion of the windings against the rotational forces of the rotor.
In accordance with an exemplary embodiment of the present invention, an electric machine comprises a rotor, a rotor winding disposed on the rotor having a body portion and end turn portions, and a non-metallic cylinder arranged around the rotor to restrain both the body portion and the end turn portions of the winding against forces resulting from a rotation of the rotor. The cylinder is made of a graphite epoxy composite and is a single integral cylinder. The rotor includes a body portion which extends along a rotational axis of the rotor and two end faces which axially oppose one another, and the cylinder extends along the rotational axis of the rotor so that the cylinder surrounds the body portion of the rotor and extends axially beyond both of the end faces and the end turn portions of the rotor winding. The electric machine of the exemplary embodiment overcomes the above problems encountered with the locking key arrangement and eliminates the need for the body wedges and corresponding slot to restrain the body portion of the windings to the rotor against its rotational forces. The elimination of the body wedges allows the amount of material forming the winding in each rotor slot to be increased.
The cylinder has a plurality of holes defined therethrough comprising a plurality of ventilation holes and a plurality of balance plug holes. The rotor includes a plurality of slots formed therein for receiving the winding. The plurality of ventilation holes are respectively positioned with respect to a plurality of ventilation through-holes in the winding to form a plurality of ventilation passages. A fan is arranged at an axial end of the cylinder so that the winding can be cooled through the venting passages. The rotor also includes at least one opening formed therein. The opening is aligned with one of the plurality of balance plug holes to allow a balance plug to be engaged into the opening through the one of the plurality of balance plug holes. The rotor includes a groove on its outer periphery and the cylinder includes a protrusion on its inner periphery which engages the groove of the rotor. This allows the cylinder to be effectively secured to the rotor. The electric machine of the exemplary embodiment thus provides the following benefits: (1) the ability to directly cool rotor end windings with increased cooling efficiency and lower cost; (2) decreasing the end turn spindle length of the rotor (by, e.g., five to seven inches) since space for cooling is no longer required; (3) a tighter end turn portion area resulting in lower core end losses; (4) simplified end turn portion blocking; (5) increased pole area for the same amount of copper forming the rotor winding; (6) eliminating the use of retaining ring insulation, ammortissuer winding and creepage blocks (e.g., up 250 parts per rotor); and (7) simplified rotor machining so that no dovetails and potentially no slots are required.
In a further exemplary embodiment of the present invention, a method of forming an electric machine comprises providing a rotor, disposing a rotor winding on the rotor, the rotor winding having a body portion and end turn portions, and arranging a non-metallic cylinder around the rotor to restrain both the body portion and the end turn portions of the winding against forces resulting from a rotation of the rotor. The cylinder is made of a graphite epoxy composite and is a single integral cylinder. The rotor is provided to have a body portion which extends along a rotational axis of the rotor and two end faces which axially oppose one another, and the cylinder is arranged such that the cylinder extends along the rotational axis of the rotor to surround the body portion of the rotor and extends axially beyond both of the end faces and the end turn portions.
The method of the exemplary embodiment further includes defining a plurality of holes through the cylinder comprising a plurality of ventilation holes and a plurality of balance plug holes. A plurality of slots are formed in the rotor for receiving the winding. The plurality of ventilation holes are positioned with respect to the plurality of ventilation through-holes in the winding to form a plurality of ventilation passages and a fan is arranged at an axial end of the cylinder to cool the winding. At least one opening is formed in the rotor and aligned with one of a plurality of balance plug holes. A balance plug is then engaged into the opening through the one of the plurality of balance plug holes. A groove is formed on an outer periphery of the rotor and a protrusion on an inner periphery of the cylinder is engaged with the groove so that the cylinder can be secured on the rotor.
In a further exemplary embodiment of the present invention, an electric machine comprises a rotor, a rotor winding disposed on the rotor, the rotor winding having a body portion and end turn portions, and a non-metallic cylinder arranged around the rotor to restrain both the body portion and the end turn portions of the winding against forces resulting from a rotation of the rotor, wherein no portion of an inner circumference of the cylinder is smaller than an outer circumference of the rotor. In another exemplary embodiment of the present invention, a method of forming an electric machine comprises providing a rotor, disposing a rotor winding on the rotor, the rotor winding having a body portion and end turn portions, and sliding a non-metallic cylinder around the rotor to restrain both the body portion and the end turn portions of the winding against forces resulting from a rotation of the rotor, wherein no portion of an inner circumference of the cylinder is smaller than an outer circumference of the rotor so that the cylinder may be slid around the rotor without obstruction.